chromium/third_party/blink/web_tests/http/tests/workers/resources/shared-worker-usecounter.js

onconnect = e => {
  var port = e.ports[0];
  port.onmessage = (e) => {
    if (e.data.type == 'COUNT_FEATURE')
      internals.countFeature(e.data.feature);
    else if (e.data.type == 'COUNT_DEPRECATION')
      internals.countDeprecation(e.data.feature);
  };
  port.postMessage('CONNECTED');
}